The present invention relates generally to solid-state lasers which utilize one or more etalons to control the frequency of the laser output and, more particularly, to a solid-state laser and method wherein an active etalon containing an active laser medium provides highly effective frequency filtering of the laser light, provides improved polarization and frequency stability of the laser light, provides improved efficiency of nonlinear conversion and provides improved modulation response of the laser.
A typical solid-state laser is capable of lasing in multiple longitudinal modes. The average frequency of these modes is determined primarily by the spectral gain distribution of the laser medium. Currently, laser oscillations are confined to a single longitudinal mode by inserting a pair of parallel reflective surfaces, known in combination as a Fabry-Perot etalon, between the front and back ends of the laser cavity to form a secondary resonant cavity. The etalon typically consists of a passive material, such as quartz, having parallel reflective surfaces.
The etalon is tilted with respect to the cavity axis such that reflections from the etalon do not interfere with the desired lasing radiation. At most wavelengths, the etalon reflects light out of the laser cavity, increasing cavity loss and suppressing laser oscillation. Light at certain wavelengths, however, experience interference effects which nullify the reflection, thus allowing laser oscillation. The frequency filtering function of an etalon is widely understood in the art and is described, for example, in Optics, Second Edition, by Eugene Hecht, Addison-Wesley Publishing Company 1987, at pages 368-371.
The etalon is designed to allow laser oscillation at wavelengths which are spaced far enough apart that only a single wavelength falls under the gain curve of the laser. Consequently, the laser oscillates at a single frequency. This oscillation frequency may be varied by turning the etalon in the laser cavity.
One common solid-state laser is the intracavity frequency-doubled laser which uses a frequency doubling medium to double the frequency of the laser light generated by the laser medium. The intracavity frequency-doubled solid-state laser is unfortunately prone to chaotic amplitude fluctuations due to nonlinear loss introduced by the intracavity frequency doubling medium. T. Baer, "Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers," J. Opt. Soc. Am. B, Vol. 3, September 1986, pp. 1175-1179. Large amplitude fluctuations due to longitudinal mode coupling are also experienced in diode-pumped intracavity doubled neodymium-doped yttrium aluminum garnet (YAG) lasers. In addition, variations in the index of refraction and the length of the doubling medium due to actual physical displacement of the medium or temperature fluctuations in the laser cavity can alter the polarization of the laser modes within the laser cavity. These unwanted changes in polarization decrease the conversion efficiency of the frequency doubling medium. The upper limit of frequency response of such lasers to direct modulation is a function of both the cavity decay rate and the level of pumping above threshold. With common solid-state laser systems, cavity decay rates are low, and threshold pump levels relatively high, allowing system performance to reach only several tens of kilohertz.
Accordingly, it is seen that a need exists in the art for a solid-state laser and method incorporating an active etalon which efficiently filters unwanted frequencies in the laser cavity, which improves the polarization and frequency stability of the laser light in the laser cavity, which improves the efficiency of nonlinear conversion in a frequency-doubled laser and which increases the modulation response of the laser.